1. Field of the Invention
The present invention relates to coal pulverizers and, more particularly, to the on-line control of the distribution of coal among the pulverized coal outlet pipes in pulverizers using independently adjustable flow control elements installed inside the pulverizer upstream of the entrance to each pulverized coal outlet pipe.
2. Description of the Background
Coal fired boilers utilize pulverizers to grind coal to a desired fineness so that it may be used as fuel for the boilers. Typically, raw coal is fed through a central coal inlet at the top of the pulverizer and falls by gravity to the grinding area. Once ground (different types of pulverizers use different grinding methods), the pulverized coal is transported upwards, using air as the transport medium. The pulverized coal passes through classifier vanes within the pulverizer. These classifier vanes may vary in structure, but are intended to establish a swirling flow within the rejects cone to prevent coarse coal particles from flowing into the discharge turret of the pulverizer. These vanes are often adjustable mechanisms. The centrifugal force field set up in the rejects cone forces the coarse coal particles to drop back down onto the grinding surface until the desired fineness is met. Once the coal is ground finely enough, it enters the discharge turret. From the discharge turret the pulverized coal is distributed among multiple pulverized coal outlet pipes and into respective fuel conduits where it is carried to the burners. Each coal pulverizer is an independent system and delivers fuel (pulverized coal) to a group of burners.
Poor balance of pulverized coal distribution between pulverized coal outlet pipes is commonly experienced in utility boilers. This can be due to various reasons, such as system resistance of each individual fuel conduit, physical differences inside the pulverizer, and coal fineness. Unbalanced distribution of coal among the pulverized coal outlet pipes adversely effects unit performance and leads to decreased combustion efficiency, increased unburned carbon in fly ash, increased potential for fuel line plugging and burner damage, increased potential for furnace slagging, and irregular heat release within the combustion chamber. In addition, it is critical for low NOx (Nitric Oxides) firing systems to precisely control air-to-fuel ratios in the burner zones to achieve low levels of NOx formation.
Therefore, there is a need in the industry for a method and apparatus that provides for on-line balance and control of the distribution of pulverized coal between the multiple pulverized coal outlet pipes of coal pulverizers.